The conventional materials used for making valve seats include cast iron, cast steel, heat-resistance steel, non-ferrous alloys and sintered alloys. A wide variety of sintered alloys with different characteristics have been developed. Use of these conventional sintered alloys, however, yields unsatisfactory results in most cases with lead-free gasoline, though good results are obtained when the gasoline contains an adequate amount of such anti-knock additives as tetraethyl lead.
Various organic leads added to the gasoline as anti-knocking agents turn into lead oxides when the gasoline burns and, when deposited on the valve and valve seat surface, they serve to protect and lubricate the valve seat or absorb the energy of valve impact, thereby preventing wear of the valve seat, but when lead-free gasoline is used, the wear-preventing effect of lead is absent and accordingly the valve seat suffers heavy wear. During use of a high-octane gasoline with much tetraethyl lead, great quantities of the products of combustion are deposited on the valve seat surface and are likely to cause heavy oxidation and lead corrosion on the valve seat of conventional materials. At the same time, as the result of a temperature rise in the exhaust system of an internal combustion engine provided with anti-emission equipment for the prevention of air pollution, the heat load of the exhaust gas on the valve seat increases and conventional materials which lack heat-and-wear resistance cannot stand up under severe operating conditions of the engine. Thus the valve seat materials have come to be required to possess higher resistances to wear, oxidation and lead corrosion and be able to stand up under severe operating conditions.
Furthermore, a valve seat, which has been pressed into a cast iron cylinder head in a conventional manner, is liable to drop out when subjected to a heavy heat load. Thus the valve seat material is required to have a lower coefficient of thermal expansion.